Use `@available` to Deprecate Constructs in Swift

[InsertCreativityHere]

It seems like the preferred way to mark something as deprecated in Swift is with:

@available(*, deprecated, message: "do not use this!")

It would be good if slice2swift could generate these on Slice definitions marked with the deprecated metadata. (Right now slice2swift completely ignores this metadata.)

There's alot of different posts and pages talking about this, but here's one of them: https://www.hackingwithswift.com/example-code/language/how-to-use-available-to-deprecate-old-apis